Method and means for continuous casting employing compartmented molds



INVEN TOR. KARL HEINZ STEIGERWALB BY fw .f4/mw um W ATTORNEYS 'f Il;

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7 IA, w c Ililll 2 EMPLOYING COMPARTMENTED MOLDS K. H. STEIGERWALD METHOD AND MEANS FOR CONTINUOUS CASTING April 16, 1963 Filed Dec. 2. 1959 prll 16, 1963 K. H.s11:|GERwALD 3,085,303

METHOD AND MEA FOR NTINuoUs CASTING EMPLOYING MPA ENTED MoLDs Filed Dec. 2. 1959 4 Sheets-Sheet 2 ifyf/ u 3e y// W@ INVENT KARL HEINZ STEM: wAu

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METHOD AND MEANS FOR coNTINuous CASTING EMPLOYING COMPARTMENTED MoLDs 4 Sheets-Sheet 3 l Filed Dec. 2. 1959 A. W 'IW .Pq //7// z 2J 7 v w ///7//// ,/vvvvv f w A all INVENTOR. KARL HEINZ. STEIQERWALU BY f 3 a l f I I Tl ID |I ///////4 A, /l 4 2 a b M \\\\\\ofw .Md F

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States dce 3,ti85,303 METHQD AND MEANS FR CNTHNUUS SAST NG EMPLYNG COMPARTMENTED MLDS Kani Heinz Steigerwald, Sauerhruch-trasse lil, Heidenheim, Vtle'rnrany Filed Dec. 2, i959, Ser. No. 856,760 8 Claims. (Cl. 22-139) This invention relates to the continuous casting of liquid metal, and more particularly, to continuous casting in which heat ilow between the mold and the casting is improved by the mold configuration.

The continuous casting process is known to the art. ln a typical process, liquid metal is cast into a continuous strand by a vertical bottomless mold. The walls of the mold are cooled so that the heat is removed from the liquid metal by the wall and the metal solidities from the .exterior to the interior in a strand. The resulting strand or casting is drawn from the mold.

The shrinkage of the metal during soliditication of the casting causes separation `of the casting skin from the mold walls in comparatively short time. After separation of the skin from the mold walls, the heat ilow between the surface of the casting and the mold is substantially reduced. Reduction of the rate of heat transfer limits the casting speed. Lengthening of the mold beyond the minimum safe length (the length necessary to ensure solidiiication of the shell of the casting to a thickness sufcient to withstand the hydrostatic pressure of the enclosed molten liquid) does not substantially increase the cooling effect and, thus, the production rate. This can be easily understood by recognition of .the fact that the separation between the casting skin and the mold walls is the basic cause of any decrease of heat transfer.

Although the decrease in rate of casting due to such separation is observable in all rcasting processes, it is particularly disturbing when large castings are made. In the casting of large strands or slabs the change of heat How has even greater influence because the maximum possible casting rate is essentially an inverse function of the cross-sectional area of the casting.

Also, since the solidiiic-ation of the liquid mass proceeds from the exterior to the interior of the casting, the crystal growthis similarly oriented. The forma-tion of dendritic crystals oriented to point to the strands center adversely affects the properties of the cast strand, particularly the resistance of the strand to transverse bending.

Moreover, the shrinkage of the core material as the strand solidiies from the 4skin to the interior often forms pipes and develops stresses and cracks inthe casting. Such faults are particularly prejudicial to the utilization of such castings 'for engineering construction.

The art h-as attempted to obt-ain more uniform cooling across the entire cross section of they casting by emersion of cold materials of the same composition as the molten metal in the melt yas it is poured into the continuous casting mold. These attempts have not, however, given satisfactory results. i

It is, therefore, one object of this invention to provide -a continuous casting method and apparatus in which the casting rate is increased over that possible with apparatus of prior art.

It is a further object of this invention to provide a continuous casting method and apparatus in whichV the cast strand is cooled at the interior and the exterior.

it is a fur-ther object of this invention to provide an improved method and apparatus for the continuous casting of metal in which the strand exhibits a more favorable crystallization.

lt is a-further object of this invention to provide a con- .,tinuous casting moldin which the cast strand skin is solidilied in axially extending sections separated by molten may be piped directly to the individual chambers. Aprevent solidiiication `of material within such piping, the

metal to maintain physical cont-act of the skin with the mold over the entire mold length.

It is a `further object of this invention to provide an improved method and means for the casting of the metal using a multiple chamber mold of increasing crosssectional areain which the chambers are iilled with molten metal to` progressively increase the cross Vsection'of the strand.

In accordance with these objects there is provided in a preferred embodiment of this invention 4a mold having a plurality of contiguous mold chambers of increasing cross sectional area. The strand is thus builtup -in stages and at each stage the central strand formed by the preceding stage serves yas a path for heat transfer to cool the molten metal inaddition to the cooling-of the metal by the mold walls. The decreased length of path for heat transfer `over the heated sections favorably influences the strands of cross sectional shape which have been ditlcult to obtain by previously known methods such as round strands. Further, the control makes feasible casting of a round strand out of two or more chambers, the -cross Asectional areas of which need not be circular in shape.

In the use of such multiple molds it is possible to decrease the length of the crater formed in the top of the cast strand. The shortening of the crater and increase of apex angle thereof greatly reduces the danger of piping even at high rates of casting.

To form the casting it -is necessary to supply molten metal to each of the chambers to ensure filling of the chambers to develop the strand of the desired cross sectional dimensions. The necessary supply of molten metal may advantageously be supplied to each chamber of increased cross sectional area by providing axially extending strips of poor thermal conductivity and good heat resist-ance in the preceded section. The strand will not solidify along the strip, providing a passage `for the iiow of molten material into the next mold chamber. If necessary, the strip may be artificially heated to maintain the passage-Way in molten form.

By positioning the strips on diametrically opposed wall portions, the metal in the mold solidifies in -axially extending skin sections which are separated by molten metal.

Thus, the solidified skin portions remain in contact with the mold walls `for efficient heat transfer thereto. No connecting skin exists Ybetween the solidified skin portions to pull Ithe solidified skin lfrom the mold walls. For example, ir" the strips are positioned along the midpoint of the end walls of a rectangular mold, the skin solidified along the side walls will remain in contact with the side walls since the side wall portions are not the mold walls until the strand leaves the mold.

In another embodiment of this invention the liquid metal in the chambers of increased cross sectional area To piping should be made of material having a poor thermal conductivity and high heat resistance.

`In a still further embodiment of this invention at least one mold wall is maintained sutliciently short to preclude solidication of the material in contact therewith. By this means, the liquid metal is supplied to subsequent chambers directly and the inserts Ioaf-heat resistant material of poor thermal conductivity is not necessary.

lIn all embodiments, it is advisable to make the difterence of cross sectional yarea between the separate chambers as small as possible. In such manner filling of the space resulting from the shrinkage of .the strand is ensured bythe additional molten material flowing therein and at the same time a continuous casting is produced with no discontinuities at the junction between the original strand and the added material.

=It should be noted that in each embodiment the separate mold chambers may be symmetrical or unsymmetrical one to the other. Additionally, the various steps in a mold chamber can have similar or different shapes.

The invention will be more clearly understood by reference to the following description ltaken in conjunction with the accompanying drawings of which:

FIGURE 1 -is a schematic view in cross section of a continuous casting process in accordance with the prior art.

FIGURE 2 is a diagrammatic view of a two-stage continuous casting mold in accordance with the present invention of which FIGURE 2a is a partially sectioned perspective view and FIGURE 2b is a sectional view taken along lines B-B of FIGURE 2; FIGURE 2c is a sectional view taken along lines C-C of FIGURE 2a; 2d is a sectional view taken along lines D-D of FIG- URE 2b; and FIGURE 2e is a rsectional view taken along lines E-E of FIG. 2b.

FIGURE 3 is a diagrammatic View of a multiple stage mold according to the present invention in which FIG- URE 3a is -a vertical sectional view; FIGURE 3b is a sectional view taken along lines 'II-II of FIGURE 3a; FIGS. 3c3f are sectional views of the mold taken along lines c-c, d-d, e-e and f-f respectively; and FIGS. 3g-3k are sectioned views or the casting taken along lines g-g, ih-h, i `and k--k respectively; and FIGS. 3l-3o are sectional views of the casting taken along lines g-g, h-h, i-i and k--k respectively in a different state of solidification.

FIGURE 4 is a diagrammatic view of a two-stage mold with the contiguous chambers arranged unsynnnetrically in which FIGURE 4m is a vertical section through a twostage mold and FIGURE 4b is a sectio nthrough lines B-B of EIGURE 4a.

FIGURE 5 is a `diagrammatic view of a two-stage mold with a square and circular contiguous chamber in which FIGURE 5a is a vertical section, 5b is a section taken along lines b--b of FIGURE 5a and FIGURE 5c is a section taken along lines c-c of FIGURE 5a.

FIGURE 6 is a sectional view of a two-stage mold in accordance with the present invention in which the mold chambers are movable relative one to the other.

FIGURE 7 is a sectioned view of a two-stage rnold having an opening in a mold wall Ibetween two mold chambers.

FIGURE 8 is a sectional view of a two-s-tage mold with the steps partially tted one into the other, in which FIG. 8a is a plan view of the top of the mold and FIG. 8b is a sectional view .taken along line b--b of FIG. 8a.

FIGURE 9 is a sectional view of a two-stage mold #for a continuous casting of a slab in which two tmold chambers jointly `feed `a parallel second mold chamber of larger cross sectional area in which FIGURE 9a is a vertical section and FIGURE 9b is a section taken along lines G-G of FIGURE 9a.

FIGURE 10 is a cross sectional view of a mold step.

FIGURE ll is a diagrammatic lsectioned view of a mold in accordance with the present invention including means ttor evacuating the mold chambers.

FIGURE l2 is a cross sectional View of a mold having axially extending inserts dimensioned to protrude into the casting cavity.

FIGURE 13 is a partially sectioned View of a mold chamber having an insert for channeling the molten metal from one mold chamber tothe next mold chamber in which FIGURE `13a is a cross sectional view along the axis of the mold and FIGURE 13b is a cross sectional View taken along line B--lB of 13a.

FIGURE 14 is a cross sectional view of a mold charnber separated into areas of smaller cross sectional area by intersecting inserts of high heat resistance and poor thermal conductivity;

FIGURE 15 is a schematic diagram of a three-step cascade mold with obliquely cu-t steps between the mold `chambers in which FIGURE 15a is a cross sectional view taken axially along -the mold and FIGURE 15b is a plan view of the top of the mold.

FIGURE 16 shows a cascade mold with a spirally expanding shaft in which FIGURE 16a is a cross sectional view taken along the axis of the mold and FIG- URE l'6b is a plan View of the t-op of the mold.

yIn FIGURE l there is shown a conventional continuous castin-g mold in which the liquid metal 11 is poured into the rnold 12. In the mold, heat is removed 'from the molten metal by the water cooled walls of the mold. As the heat is removed from the metal, the metal solidifies into a completely or partially solidied cylindrical strand 13 and leaves the mold. The strand or casting is pulled Ifrom the lmold by conventional means such as Withdrawal rollers. Such equipment and rthe process of casting therewith is well known and only the few characteristics of interest are illustrated in FIGURE 1.

When the liquid meta-l comes into contact with the cooled wall of the mold, a self-containing shell of partially solidified metal is vquickly formed about the periphery of the cylindrical casting. The cylindrical shell must be lsufficiently thick when it leaves the mold so as to contain the hydrostatic head of the molten material contained thereby.

Of course, initially the shell Iis -in close contact with the wall, since the liquid material flows until it is restrained by the mold Walls. However, subsequently, the lstrengthened shell of the solidified material shrinks and parts from the mold as at line 19.

Thus, a casting is in contact with the mold only over a small portion (between lines 18 and 19) of the total mold axial length. In this area, of course, effective cooling takes place to form the skin of the mold. However, as the casting passes below line 19 shrinkage of the skin of the strand causes a gap to exist between the strand skin and the mold wall. This gap causes a noticeable decrease in hea-t transfer between the casting and the mold wall. For this reason, elongation of the mold in an axial direction does not produce any noticeable gain in the -rate of heat transfer. Thus the casting speed with such molds is limited.

Further, it is typical of such processes of continuous casting that a crater 20 of generally sharply conical shape tilled with molten metal forms in the center of .the cast strand. The shape and length of the crater depends essentially upon the following factors; (a) the thermal properties of the cast metal, (b) the cooling properties of the mold, (c) the size of the cross sectional area and the shape of the mold cross section, and (d) the rate of the casting.

In addition to the influence of the crater upon the rate of casting in Such continuous casting process, the shape and Ilength inuences the quality of the casting. Since the conical crater contains molten metal, the conical section has the material of the highest temperature on the inside of the casting dropping to a minimum temperature at the outside. Cooling of the casting in this Ifashion produces a crystallization which in many cases inhibits the use of continuous cast columns for structural purposes. Further, the crater lformation leads to other faults such as pipe, porosity and cracking.-

Unfortunately, the failure rate and the limits of rates of casting by the continuous casting process become increasingly aggravated Vas the cross section of the casting increases and as the rate of casting increases.

The faults associated with casting according to the prior art is obviated by cool-ing the casting Ifrom the interior thereof as well as cooling the casting by the heat transfer to the mold walls. An apparatus in accordance with this invention -for so effecting the cooling of fthe casting is shown in FIGURE 2.

In FIGS. 2a-2e there is shown a continuous casting composite mold for the vertical casting of strands of molten metal poured therein. The composite mold cornprises two contiguous concentric molds or mold chambers 30 and 31.

As in the lformation of molds common to the art of continuous casting, the mold shown in FIGURE 2 may be fabricated lfrom copper and water cooled in conventional fashion `as by-water cooling ports 16.

The mold shaft of chamber 30 and shaft of the mold 31 are both formedwith a square cross sectional shape by walls 32, 33` respectively. rIhe area of shaft 32 is approximately 1/3 of the area of the cross section of shaft 33.

On diametrically opposed wall portions of walls 32 of the shaft in mold 30, axially extending inserts 34, 35 are provided. The inserts are fabricated of material having good heat resistance but poor thermal conductivity.

=In operation, molten metal 36 is poured into the mold and it is cast into a strand by mold walls 32 of mold chamber 30. Continuous movement of the cast strand will cause the strand to be moved into the mold chamber 31. In the mold chamber 31 additional metal is cast on the periphery of the strand formed by chamber 30l to increase the cross -sectional area of the resultant strand. During such addition the strand formed by chamber 30 will serve as a heat conducting path to cool the interior of the strand cast Iby mold chamber 31 eliminating the faults encountered by exterior cooling alone.

To Isupply `the chamber 31 with the molten metal in order to till the chamber to the periphery thereof, there must be provided means for conducting molten material lfrom the entrance port `of the mold chamber 30 to the mold chamber 31.

The inserts 34, 35 extending axially along the walls of the mold chamber 31. serve this purpose by preventing solidication of the material into a skin along the axially extending strips. As best seen by reference to FIGURES 2b and 2d simultaneously the axially extending strips of material having .poor thermal conductivity prevents solidification of the skin. Thus, Vthere is provided a path for the iow of molten metal from chamber 30 to the chamber 31.

\As the casting moves through the mold, the skin is first formed in axially extending sections 39 by the cooled Walls of the mold. The skin sections 39 are separated by molten metal extending .between the axially extending strips 34 and 35 since the poor thermal conductivity of the axially extending strips prevents solidiiication of the molten metal. As` the strand progresses through the :mold chamberit) it is solidified into a strand 43 with axially extending passages 45 for the ow of molten metal to the chamber 31. i

In Vthe second mold chamber 31, the strand is immersed in the molten metal 57 flowing therein through the passages in the strand formed in the first chamber. The metal 57 will solidify into the strand of iinal dimensions by heat transfer to both .themold walls and the strand cast by the mold chamber 30.

The molten metal 57 will solidify, joining strands 39 at 52, 53, 54 and S5, to form the final strand 47. VThe molten-metal 57 forms the same crater configuration as the Crater S6 in the top of mold chamber 30.

It `will be notedthat the strand in both mold sections is `connected by solidified metal. Thus the usual withdrawal rollers may be employed to pull the completed strand from the composite mold. In many cases, the molten metal surrounding the strands 39 will cause some melting thereof as indicated at 58. The melting augments Vproper joinder at the interface and causes no diculty since the composite strand is always coupled through solidified metal.

-As will be noted from the drawings, the strand advances at the same speed through both chambers. Thus the chambers may be oscillated in the usual fashion and other casting aids such as a vacuum at the exit end or" the moldmay be employed ifl desired for the application intended.

In addition to providing an unsolidiiied path for the flow of molten metal the axially extending inserts vastly improve the rate of cooling of the cast strand. Along the axially extending strips, the strand is maintained in the molten condition. Thus, the strand skin sections 39 formed by Contact with the cooled surfaces of the mold are .not connected by solidified metal which would exert a force to pull the skin from the mold walls during shrinkage thereof. Instead, the diametrically opposed skin sections 39 are separated by molten metal and the skin sections will maintain their Contact with the cooling mold vwalls throughout the entire axial length of the mold.

It will be recognized that the process of casting by additive casting of a plurality of increasedsections upon a central strand of smaller dimension can be variously modified to suit the application intended. For example, the length of the mold step maybe varied to conform to a systematic distribution of cooling and non-cooling elements i-nside the individual mold steps.

Similarly, each mold step can be variously modied to Vsuit the application intended by variation of the arrangement of cooling and non-cooling elements in each step.

Further, it is possible, of course, to cascade more than `two steps when it is desirable to cast a strand of large finished dimensions. Control of the cooling at each step is provided by maintaining the increase in cross sections of contiguous chambers to thereby keep the path of heat transfer across the added strand portion small. An arrangement which maybe advantageously used in such applications is shown in FIGURE 3.

In FIGS. Saz-3o there is a multiple mold having four transverse steps and two longitudinal steps, as is best illustrated by FIGURES 3c-2f which are sections of tht mold taken along lines c--c, d-d, e-e and f-f respectively on FIG. 3a.

Thus, it can be seen that the strand is slowly built up from a small rectangle in gradually increasing steps to a large square strand.

During movement of the molten material through the mold chambers as explained in connection with FIGURE 2 and is illustrated by FIGURES 3g through 3k, the molten metal will solidify on the mold walls in two strand skin sections separated from each other by molten material.

By properly dimensioning the step length with respect to the increase in cross sectional area at each chamber step, the solidilication of the center of the strand will proceed without isolating a subsequent chamber from the source of molten metal. In order to provide for the iiow of molten metal, axially extending strips of heat resistant material of poor thermal conductivity are in- Vserted in each of the mold chambers except the last mold chamber.

By varying the length of each mold chamber, the solidiiication of the strand can be controlled. For example, in FIGURES 3l to 30 there is shown a plurality of sections of a strand along the same section lines as FIGURES 3g to 3k in which, however, the cooling effect in each chamber has been increased as 4for example by increase in the chamber axial length. As illustrated, the strand solidiiies faster without, however, interfering with the passage provided for iiow of molten metal to each chamber. It is, of course, necessary to maintain the passageway to prevent gas entrapment and to prevent piping.

In the embodiments Aillustrated .thus far, concentric mold chambers have been shown. However, with this process concentricity of the mold chambers is not required. Where it is desirable to provide eccentric mold chambers, as for example, for simplicity of mold construction the embodiments shown in FIGURE 4 may advantageously be employed.

In FIGS. 4a and 4b there is shown a composite mold comprising a chamber 7.1 and a chamber 72 in which the chambers are contiguous one to the other but in which the axis thereof are displaced one from the other.

By displacing the axial position of the two chambers as shown, the mold walls 73 and 74 extend as straight walls across both chambers and may be formed in a continuous length. The chamber may then be formed and supported on the continuous wall portion. An axially extending 75 of heat resistant material having poor `thermal conductivity is provided in the wall portion of the vupper mold 71 so that a passageway to mol-d 72 is provided.

The insert of heat resistant material of poor thermal conductivity is preferably placed as close to the axis of the second chamber 72 as is possi-ble to ensure the shortest `iiow distan-ce to the periphery thereof.

In many applications it is desirable that the nal stage of -the composite mold be of one cross sectional shape but that the intermediate chambers 'be of different cross sectional shapes as, for example, for the heat transfer characteristics of the shapes involved. In such applications :the embodiment shown in FIGURE 5 may advantageously be employed.

In FIGURE 5a there is shown a composite mold in which the first mold chamber, illustrated in cross section in FIGURE 5b, is square in cross sectional shape. The second mold chamber arranged contiguous thereto is oircular in cross sectional shape, illustra-ted in cross section in FIGURE 5c. In such matter the cast strand can take a cylindrical form. However, the increased cooling effect of the square mold is utilized in the initial strand formation for increased heat flow to the walls of the mold and thus a higher casting rate.

In many applications it is advantageous to have the various mold -chamber-s integrally formed in a single mold. In other applications it is desirable to fabricate the `separate chambers so that, for example, they may be oscillated with different phase relationships during `the casting process. In such applications the embodiment shown in FIGURE 6 may 'advantageously be employed.

In FIGURE 6 there is shown a mold comprising a mold chamber 76 which is movably mounted on axially extending guide rails 73 upon which the mold is mounted by -means of struts 77. A second mold chamber 79 the bore of which matches the external periphery of the chamber 76 is movably positioned in telescopic relationship to the mold chamber 76. The mold 79 is similarly movably mounted lon guide rails 78 by means of struts 80.

In such application the molds may be reciprocated in accordance with the speed of casting in conventional fashion and the phase relationship may -be adjusted to suit the casting contained therein. For example, chamber 76 may be moved in the direction of arrowr81 at the same time that mold chamber 79 is moved in the direction of arrow 82. Of course, diiferent phase relationships than a 180 phase relationship may be maintained between the oscillations of the respective mold chambers.

Between the mold steps a circular oriiice can remain Vopen or it is possible to provide 4window shaped openings at the gate between the two steps as is shown schematical- 1y lby the opening 33 in FIGURE 7. The opening 83 can port the lower mold into a shaft-like chamber formed by an extension of the lower mold illustrated in dotted outline as extension 34 -in the FIGURE 7.

In FIGS. 8a and 8b there is shown a mold comprising two cascade steps in which the chamber is defined by wall 85 and the chamber of a larger dimension dened by wall 86. To provide a passageway for ow of molten metal into the chamber of larger dimensions', the mold but poor thermal conductivity.

The molds are joined at the tops thereof by outwardly extending ribs 8'7. At the base of the small chamber there is provided a peripherally extending rectangle 89 of the insert material to join the inserts together and to completely separate the inner chamber from the outer chamber. It will be noted that the inserts are the same thickness as the mold chamber walls 85.

With the arrangement shown in FIGURE 8, it is possible to let the liquid metal in the chamber defined by walls 86 extend over the lower end 89 of the chamber 85. It has been rfound advantageous so to do in order to utilize the cooling effect of the mold wall of the chamber S5 to provide an additional heat p-ath for cooling of the material in the interior of the chamber 86. In such embodiment, the water cooling of chamber will -be provided lthrough the ribs 87.

In those applications where it is desirable to cast a large slab of material, it is often diiiicult to provide the necessaly quantity of the flow of molten metal of the mold and to provide a passageway for the molten metal to the lowest mold chamber. In such applications the embodiment shown in FIGURE 9 may advantageously be employed.

In FIGS. 9a and 9b there is shown a composite mold having paralleled mold chambers 89! and 90 to which is connected a contiguous slabv chamber 91. The two mol chambers S9 yand 90 chili the strand and are joined together in a single continuous slab defined by the periphery of the chamber 91. Flow of material into the chamber 91 may be provided by maintaining the material molten through the use of axially extending insert of poor thermal conductivity in the two chambers 89 and 90 as has been explained in connection with the explanation of FIGURES 2-8.

It will be noted that the transition between the mold chambers may be gradual as it is illustrated in connection with the embodimetnt shown in FIGURE 3. It is often simpler to 'construct the beveled connection between mold chambers in the different cascade steps and it is often found that the beveled transition secures maximum contact between the skin and the mold walls.

It has been found further that to secure the maximum heat transfer contact between the skin and the mold walls that the mold chambers are desirably maintained relatively short in axial length. A short axial length and the relatively small diiference in cross sectional area between such stages ensures that the molten metal iills the peripheral gap between the mold walls and the strand skin and at the same time ensures that there is a continuous casting of the metal with proper joining of the added peripheral castings.

In many applications it has been found that variations in the mold wall design will improve the heat transfer relationship and the guidance of the strand skins through the mold. An example of such mold is shown in FIG- URE 10.

In FIGURE yl() there is shown a mold having a projecting wall portion I1&0 within which is inserted an axially extending strip 121 of material having high heat resistance and poor thermal conductivity. The extending Wall forms a flange-shaped guide 122 for the solidified wall portions of the strand, as the portions are formed by contact with the walls of the mold separated by the inserts.

In many applications it is desirable to augment the contact between the strand skin and the wall of the mold by a vacuum therebetween. `In such applications, the embodiment shown in FIGURE 11 may advantageously be employed.

In FIGURE l1 there is shown the mold wall 92 having an aperture 93 extending therethrough. Coupled to the Vaperture is a conduit 94 to which is connected a vacuum pump in conventional manner. The lvacuum developed by the pump continuously maintains a vacuum between the skin of the casting 95 and the mold wall to force the skin into contact with the mold Wall.

In many applications it is desirable that the skin be 4formed in a plurality of axially extending sections. In .such applications [the embodiment shown in FIGURE 12 `ly extending passageways, it is equally -feasible to provide means for piping the molten metal to mold chambers of increased cross sectional area. In such applications the embodiments shown in FIGURE 13 may advantageously be employed.

In FIGS. 13a and 13b there is shown a mold having two contiguous chambers 1.91 Vand 102. In the upper chamber an axially extending insert of poor thermal conductivity 99 is provided to maintain a passageway for the fiow of material therethrough. At the termination of the insert thereis provided a pipe 160 between the upper chamber 161 andthe lower chamber 102 defined by `walls 103 and 104. 'Ifhe walls 103 and 104 are preferably coated or plated with avmaterial having good heat resistance and poor thermal `conductivity to prevent solidification of the molten metal within the pipe 100.

With such a Vmold the material cast in molten form is maintained molten in the passageway defined by the insert 99. The passage is extended by the pipe 100i to supply metal to the strand in chamber 102. By such arrangement a completely solidified strand can be cast into a larger strand by the addition of material to the periphery thereof which will solidify into contact therewith in a strand of increased cross sectional area.

In some applications it has been found desirable for the improved heat transfer of material to cast the strand in a plurality of thin strands joined together in subsequent chambers. In such applications the embodiment shown in FIGURE 14 may advantageously be employed.

In FIGURE 14 there is shown a mold 107 having axially extending inserts 105 which extend across the mold to divide the mold into four parts defined by the area A1%. In such applications the strand will comprise a plurality of small strands which Will be joined during passage through subsequent mold chambers.

It will be noted by those skilled in the art that the supply of metal to the two contiguous mold chambers may be made independently from separate or similar sources of the molten metal by pipes leading directly to the mold chambers. `In principle, of course, it is possible to cast the strands independently of one another. That is, the inner strand can be completely solidified and increases in the strand cross sectional area made by metal introduced in the subsequent mold chambers and cast to the periphery of the central strand. The fact that the central strand is in a soft and heated state allows suliicient melting thereof to ensure proper fusion between the strand periphery by the molten metal applied in the subsequent chamber.

In many applications it is desirable to provide for fiow of molten metal from the contiguous chambers by means of the design of the length of the walls of the mold charnbers. In this manner inserts of heat resistant material of poor thermal conductivity and the provision of piping may be eliminated by mold design. In such applications lf) the embodiment shown in FIGURE 15 may be advantageously employed.

In FIGS. 15a and 15b there is shown a composite mold 108 formed of three contiguous chambers v109, '110 and V111 respectively. The common edge between each mold chamber is cut obliquely.

By providing for an obliquely cut common `edge between mold chambers, it is apparent that the diametrically opposed walls of the mold chambers are of varying lengths. lIt is, therefore, possible by correct dimensioning of the wall length to ensure that the shorter wall `is insufficient to solidify themolten material into a skin Abefore reaching the second chamber 110. Thus, the

molten metal poured into the composite mold will flow into the subsequent chambers by virtue of the factthat the mold wall -is of insufficient length to solidify the molten material into askin. As will be noted particularly from FIGURE 15b the mold chambers are concentric.

lIn those applications where it is desired to form a .cylindrical strand the embodiment shown in FIGURE 16 may yadvantageously be employed.

In FIGURE 16 there is shown a composite mold comprising a chamber 112 of square cross sectional shape and a chamber contiguous thereto. The mold cham- .ber 115 is of varying cross sectional area developed by the spirally developed wall thereof. The wall expansion ,s tarts at .point l113` and continuous outwardly along a spiral until the desired circular cross sectional shape is reached. By correct dimensioning of the pitch of the spiral, filling of the enlarged mold chambers is ensured and molten metal ow to all such chambers is ensured.

It will be noted that the dimensions of the mold chambers will be governed by the thermal characteristics of the molten metal Acast as well as by the desired rate of casting of the metal.

This invention may be variously modified and embodied with-in the scope of the subjoined claims.

What is claimed is:

1. The method of continuously casting a metal strand from molten metal which comprises pouring the molten metal into a first constraining mold having a shaft to define the periphery of the molten metal as it travels through the shaft along the central axis thereof, selectively cooling the mold to form a skin on opposed discontiguous Wall portions of the shaft, each of said Wall portions being less than one-half of the wall periphery of said shaft, said formed skins being completely separated by a central core of molten metal, moving the chilled skins into a second constraining mold having a lmold shaft of greater cross section area than the shaft of said first mold, filling the shaft of said second mold by molten metal flow through said central core, cooling said second mold to form a peripheral skin defining said strand, and continuously removing said strand as formed from said second mold.

2. The method according to claim 1 which includes the step of piping the molten metal from said first constraining mold into said second constraining mold to fill the shaft thereof.

f3. The method of continuously casting a metal strand from molten metal in accordance with claim 1 which includes the step of laterally supporting said chilled skins during continuous movement through said first mold section to maintain continuous contact of said skins with the respective wall portions.

4. A composite mold for the continuous casting of a metal strand from molten metal comprising a first constraining mold having walls defining a mold shaft, said shaft constraining molten metal poured therein to define the periphery thereof as the metal passes through the shaft along the central axis thereof, inserts mounted in the walls of said shaft and extending along said central axis of said shaft, said inserts having suiiciently poor 1 l thermal conductivity as to prevent solidification of said molten metal as it passes through said mold shaft, means to cool the Walls of said first constraining mold to form a skin of solidified metal on opposed wall portions separated by a central core of molten metal extending into contact with said inserts, a second constraining mold having walls defining a second mold shaft having a larger cross sectional area than the mold shaft of said first mold, said second mold `shaft positioned contiguous with said rst mold shaft, and means tocool the Walls of the shaft of the second mold to form a strand from the solidified skins formed by the tlirst mold and the molten metal owing through the molten central core in said first mold and into the second mold.

5. A mold in accordance with claim 4 which includes pipe means 4for transferring molten metal from said central core in said first mold to the mold shaft of said second mold.

`6. A composite mold in accordance with claim 4 in which said first mold is movably mounted for movement along said central axis thereof, in which said second mold is movably mounted for movement along the central axis thereof, and in which said first and second molds are positioned in telescopic relationship.

7. A composite mold `for the continuous casting of a metal strand from molten metal comprising a first constraining mold having a Wall defining a mold shaft, said shaft constraining lthe metal to define the periphery thereof as it passes through the mold along the central axis of said shaft, one portion of said Wall having a shorter length in the direction of said central axis than the remaining portion of the wall, means to cool the walls of said shaft suiiiciently rapidly with respect to the rate of movement through said shaft to form a skin of said metal on the surface of said Wall encircling a molten core except at that portion of the wall having a shorter length, a second constraining mold said constraining mold positoned contiguous with said first constraining mold,

.said second constraining mold having Walls defining a' mold shaft having a larger cross sectional area than the mold shaft of said first mold, and means to cool the vWalls of the shaft of the second mold to form a strand from the solidified skin formed in said first mold and the molten metal fed into the second mold through the molten central core.

8. A composite mold for the continuous casting of a strand of metal from molten metal poured into said composite mold which comprises a first mold chamber hav ing Walls defining a first shaft, a second mold chamber having Walls defining a second shaft of larger cross sectional dimension than said first shaft, said chambers being coupled together with said first shaft and second shafts in contiguous relationship, means for cooling the walls of said chambers to solidify molten metal contained by said Walls, and axially extending strips of material of poorY thermal conductivity mounted in the walls of said first shaft to prevent solidification of said molten metal along said strips thereby to provide channels of molten. metal for flow of said metal into said second shaft.

References Cited in the file of this patent UNITED STATES PATENTS 373,551 Brinell Nov. 22, 1887 1,883,382 Kinzel Oct. 18, 1932 1,961,399 Snook June 5, 1934 1,965,603 Low July 10, 1934 2,058,447 Hazelett Oct. 27, 1936 2,135,183 Junghans Nov. l, 1938 2,242,350 Eldred May 20, 1941 2,527,545 Goss Oct. 31, 1950 2,543,936 Reynolds Mar. 6, 1951 2,847,737 Brennan Aug. 19, 1958 2,858,586 Brennan Nov. 4, 1958 FOREIGN PATENTS 826,273 France Mar. 28, 1938 165,284 Great Britain June 30, 1921 

4. A COMPOSITE MOLD FOR THE CONTINUOUS CASTING OF A METAL STRAND FROM MOLTEN METAL COMPRISING A FIRST CONSTRAINING MOLD HAVING WALLS DEFINING A MOLD SHAFT, SAID SHAFT CONSTRAINING MOLTEN METAL POURED THEREIN TO DEFINE THE PERIPHERY THEREOF AS THE METAL PASSES THROUGH THE SHAFT ALONG THE CENTRAL AXIS THEREOF, INSERTS MOUNTED IN THE WALLS OF SAID SHAFT AND EXTENDING ALONG SAID CENTRAL AXIS OF SAID SHAFT, SAID INSERTS HAVING SUFFICIENT POOR THERMAL CONDUCTIVITY AS TO PREVENT SOLIDIFICATION OF SAID MOLTEN METAL AS IT PASSES THROUGH SAID MOLD SHAFT, MEANS TO COOL THE WALLS OF SAID FIRST CONSTRAINING MOLD TO FORM A SKIN OF SOLIDIFIED METAL ON OPPOSED WALL PORTIONS SEPARATED BY A CENTRAL CORE OF MOLTEN METAL EXTENDING INTO CONTACT WITH SAID INSERTS, A SECOND CONSTRAINING MOLD HAVING WALLS DEFINING A SECOND MOLD SHAFT HAVING A LARGER CROSS SECTIONAL AREA THAN THE MOLD SHAFT OF SAID FIRST MOLD, SAID SECOND MOLD SHAFT POSITIONED CONTIGUOUS WITH SAID FIRST MOLD SHAFT, AND MEANS TO COOL THE WALLS OF THE SHAFT OF THE SECOND MOLD TO FORM A STRAND FROM THE SOLIDIFIED SKINS FORMED BY THE FIRST MOLD AND THE MOLTEN METAL FLOWING THROUGH THE MOLTEN CENTRAL CORE IN SAID FIRST MOLD AND INTO THE SECOND MOLD. 